Kinetics of early molecular events in duck hepatitis B virus replication in primary duck hepatocytes

Synchronised infection identifies early rate-limiting steps in the hepatitis B virus life cycle

Hepatitis B virus (HBV) is an enveloped DNA virus that contains a partially double-stranded relaxed circular (rc) DNA. Upon infection, rcDNA is delivered to the nucleus where it is repaired to covalently closed circular (ccc) DNA that serves as the transcription template for all viral RNAs. Our understanding of HBV particle entry dynamics and host pathways regulating intracellular virus trafficking and cccDNA formation is limited. The discovery of sodium taurocholate co-transporting peptide (NTCP) as the primary receptor allows studies on these early steps in viral life cycle. We employed a synchronised infection protocol to quantify HBV entry kinetics. HBV attachment to cells at 4°C is independent of NTCP, however, subsequent particle uptake is NTCP-dependent and reaches saturation at 12 h post-infection. HBV uptake is clathrin- and dynamin dependent with actin and tubulin playing a role in the first 6 h of infection. Cellular fractionation studies demonstrate HBV DNA in the nucleus within 6 h of infection and cccDNA was first detected at 24 h post-infection. Our studies show the majority (83%) of cell bound particles enter HepG2-NTCP cells, however, only a minority (<1%) of intracellular rcDNA was converted to cccDNA, highlighting this as a rate-limiting in establishing infection in vitro. This knowledge highlights the deficiencies in our in vitro cell culture systems and will inform the design and evaluation of physiologically relevant models that support efficient HBV replication.

Comparative Transcriptomic Analysis of Primary Duck Hepatocytes Provides Insight into Differential Susceptibility to DHBV Infection

Primary duck hepatocytes (PDH) displays differential susceptibility to duck hepatitis B virus when maintained in the media supplemented with fetal bovine serum or dimethyl sulfoxide (DMSO) which has been widely used for the maintenance of hepatocytes, and prolonging susceptibility to hepadnavirus. However the mechanism underlying maintenance of susceptibility to hepadnavirus by DMSO treatment remains unclear. In this study, a global transcriptome analysis of PDHs under different culture conditions was conducted for investigating the effects of DMSO on maintenance of susceptibility of PDH to DHBV in vitro. The 384 differential expressed genes (DEGs) were identified by comparisons between each library pair (PDHs cultured with or without DMSO or fresh isolated PDH). We analyzed canonical pathways in which the DEGs were enriched in Hepatic Fibrosis / Hepatic Stellate Cell Activation, Bile Acid Biosynthesis and Tight Junction signaling. After re-annotation against human genome data, the 384 DEGs were pooled together with proteins belonging to hepatitis B pathway to construct a protein-protein interaction network. The combination of decreased expression of liver-specific genes (CYP3A4, CYP1E1, CFI, RELN and GSTA1 et al) with increased expression of hepatocyte-dedifferentiation-associated genes (PLA2G4A and PLCG1) suggested that in vitro culture conditions results in the fading of hepatocyte phenotype in PDHs. The expression of seven DEGs associated with tight junction formation (JAM3, PPP2R2B, PRKAR1B, PPP2R2C, MAGI2, ACTA2 and ACTG2) was up-regulated after short-term culture in vitro, which was attenuated in the presence of DMSO. Those results could shed light on DHBV infection associated molecular events affected by DMSO.

Viral DNA-Dependent Induction of Innate Immune Response to Hepatitis B Virus in Immortalized Mouse Hepatocytes

Hepatitis B virus (HBV) infects hundreds of millions of people worldwide and causes acute and chronic hepatitis, cirrhosis, and hepatocellular carcinoma. HBV is an enveloped virus with a relaxed circular (RC) DNA genome. In the nuclei of infected human hepatocytes, conversion of RC DNA from the incoming virion or cytoplasmic mature nucleocapsid (NC) to the covalently closed circular (CCC) DNA, which serves as the template for producing all viral transcripts, is essential to establish and sustain viral replication. A prerequisite for CCC DNA formation is the uncoating (disassembly) of NCs to expose their RC DNA content for conversion to CCC DNA. We report here that in an immortalized mouse hepatocyte cell line, AML12HBV10, in which RC DNA exposure is enhanced, the exposed viral DNA could trigger an innate immune response that was able to modulate viral gene expression and replication. When viral gene expression and replication were low, the innate response initially stimulated these processes but subsequently acted to shut off viral gene expression and replication after they reached peak levels. Inhibition of viral DNA synthesis or cellular DNA sensing and innate immune signaling diminished the innate response. These results indicate that HBV DNA, when exposed in the host cell cytoplasm, can function to trigger an innate immune response that, in turn, modulates viral gene expression and replication.

IMPORTANCE

Chronic infection by hepatitis B virus (HBV) afflicts hundreds of millions worldwide and is sustained by the episomal covalently closed circular (CCC) DNA in the nuclei of infected hepatocytes. Release of viral genomic DNA from cytoplasmic nucleocapsids (NCs) (NC disassembly or uncoating) is a prerequisite for its conversion to CCC DNA, which can also potentially expose the viral DNA to host DNA sensors and trigger an innate immune response. We have found that in an immortalized mouse hepatocyte cell line in which efficient CCC DNA formation was associated with enhanced exposure of nucleocapsid-associated DNA, the exposed viral DNA indeed triggered host cytoplasmic DNA sensing and an innate immune response that was able to modulate HBV gene expression and replication. Thus, HBV can, under select conditions, be recognized by the host innate immune response through exposed viral DNA, which may be exploited therapeutically to clear viral persistence.

The Molecular and Structural Basis of HBV-resistance to Nucleos(t)ide Analogs

Infection with hepatitis B virus (HBV) is a worldwide health problem. Chronic hepatitis B can lead to fibrosis, liver cirrhosis, and hepatocellular carcinoma (HCC). Management of the latter two conditions often requires liver transplantation. Treatment with conventional interferon or pegylated interferon alpha can clear the virus, but the rates are very low. The likelihood, however, of viral resistance to interferon is minimal. The main problems with this therapy are the frequency and severity of side effects. In contrast, nucleos(t)ide analogs (NAs) have significantly lower side effects, but require long term treatment as sustained virological response rates are extremely low. However, long term treatment with NAs increases the risk for the development of anti-viral drug resistance. Only by understanding the molecular basis of resistance and using agents with multiple sites of action can drugs be designed to optimally prevent the occurrence of HBV antiviral resistance.

Differential gene expression analysis of in vitro duck hepatitis B virus infected primary duck hepatocyte cultures

Background

The human hepatitis B virus (HBV), a member of the hepadna viridae, causes acute or chronic hepatitis B, and hepatocellular carcinoma (HCC). The duck hepatitis B virus (DHBV) infection, a dependable and reproducible model for hepadna viral studies, does not result in HCC unlike chronic HBV infection. Information on differential gene expression in DHBV infection might help to compare corresponding changes during HBV infection, and to delineate the reasons for this difference.

Findings

A subtractive hybridization cDNA library screening of in vitro DHBV infected, cultured primary duck hepatocytes (PDH) identified cDNAs of 42 up-regulated and 36 down-regulated genes coding for proteins associated with signal transduction, cellular respiration, transcription, translation, ubiquitin/proteasome pathway, apoptosis, and membrane and cytoskeletal organization. Those coding for both novel as well as previously reported proteins in HBV/DHBV infection were present in the library. An inverse modulation of the cDNAs of ten proteins, reported to play role in human HCC, such as that of Y-box binding protein1, Platelet-activating factor acetylhydrolase isoform 1B, ribosomal protein L35a, Ferritin, α-enolase, Acid α-glucosidase and Caspase 3, copper-zinc superoxide dismutase (CuZnSOD), Filamin and Pyruvate dehydrogenase, was also observed in this in vitro study.

Conclusions

The present study identified cDNAs of a number of genes that are differentially modulated in in vitro DHBV infection of primary duck hepatocytes. Further correlation of this differential gene expression in in vivo infection models would be valuable to understand the little known aspects of the hepadnavirus biology.

Evaluation of the minimal replication time of Cauliflower mosaic virus in different hosts

Though the duration of a single round of replication is an important biological parameter, it has been determined for only few viruses. Here, this parameter was determined for Cauliflower mosaic virus (CaMV) in transfected protoplasts from different hosts: the highly susceptible Arabidopsis and turnip, and Nicotiana benthamiana, where CaMV accumulates only slowly. Four methods of differing sensitivity were employed: labelling of (1) progeny DNA and (2) capsid protein, (3) immunocapture PCR,, and (4) progeny-specific PCR. The first progeny virus was detected about 21 h after transfection. This value was confirmed by all methods, indicating that our estimate was not biased by the sensitivity of the detection method, and approximated the actual time required for one round of CaMV replication. Unexpectedly, the replication kinetics were similar in the three hosts; suggesting that slow accumulation of CaMV in Nicotiana plants is determined by non-optimal interactions in other steps of the infection cycle.

Intrahepatic expression of genes affiliated with innate and adaptive immune responses immediately after invasion and during acute infection with woodchuck hepadnavirus

The importance of effective immune responses in recovery from acute hepadnaviral hepatitis has been demonstrated. However, there is no conclusive delineation of virological and immunological events occurring in the liver immediately after hepadnavirus invasion and during the preacute phase of infection. These very early events might be of primary importance in determining the recovery or progression to chronic hepatitis and the intrinsic hepadnaviral propensity to persist. In this study, applying the woodchuck model of acute hepatitis B, the hepatic kinetics of hepadnavirus replication and activation of genes encoding cytokines, cytotoxicity effectors, and immune cell markers were quantified in sequential liver biopsies collected from 1 h postinoculation onward by sensitive real-time cDNA amplification assays. The results revealed that hepadnavirus replication is established in the liver as early as 1 hour after infection. In 3 to 6 h, significantly augmented intrahepatic transcription of gamma interferon and interleukin-12 were evident, suggesting activation of antigen-presenting cells. In 48 to 72 h, NK and NKT cells were activated and virus replication was transiently but significantly reduced, implying that this early innate response is at least partially successful in limiting virus propagation. Nonetheless, T cells were activated 4 to 5 weeks later when hepatitis became histologically evident. Collectively, our data demonstrate that virus replication is initiated and the innate response activated in the liver soon after exposure to a liver-pathogenic dose of hepadnavirus. Nevertheless, this response is unable to prompt a timely adaptive T-cell response, in contrast to infections caused by other viral pathogens.

Formation of hepatitis B virus covalently closed circular DNA: removal of genome-linked protein

Hepatitis B virus (HBV) contains a small, partially double-stranded, relaxed circular (RC) DNA genome. RC DNA needs to be converted to covalently closed circular (CCC) DNA, which serves as the template for all viral RNA transcription. As a first step toward understanding how CCC DNA is formed, we analyzed the viral and host factors that may be involved in CCC DNA formation, using transient and stable DNA transfections of HBV and the related avian hepadnavirus, duck hepatitis B virus (DHBV). Our results show that HBV CCC DNA formed in hepatoma cells was derived predominantly from RC DNA with a precise junction sequence. In contrast to that of DHBV, HBV CCC DNA formation in cultured cells was accompanied by the accumulation of a RC DNA species from which the covalently attached viral reverse transcriptase (RT) protein was removed (protein-free or PF-RC DNA). Furthermore, whereas envelope deficiency led to increased CCC DNA formation in DHBV, it resulted mainly in increased PF-RC, but not CCC, DNA in HBV, suggesting that the envelope protein(s) may negatively regulate a step in CCC DNA formation that precedes deproteination in both HBV and DHBV. Interestingly, PF-RC DNA, in contrast to RT-linked RC DNA, contained, almost exclusively, mature plus-strand DNA, suggesting that the RT protein was removed preferentially from mature RC DNA.

Superinfection exclusion in duck hepatitis B virus infection is mediated by the large surface antigen

Superinfection exclusion is the phenomenon whereby a virus prevents the subsequent infection of an already infected host cell. The Pekin duck hepatitis B virus (DHBV) model was used to investigate superinfection exclusion in hepadnavirus infections. Superinfection exclusion was shown to occur both in vivo and in vitro with a genetically marked DHBV, DHBV-ClaI, which was unable to establish an infection in either DHBV-infected ducklings or DHBV-infected primary duck hepatocytes (PDHs). In addition, exclusion occurred in vivo even when the second virus had a replicative advantage. Superinfection exclusion appears to be restricted to DHBV, as adenovirus, herpes simplex virus type 1, and vesicular stomatitis virus were all capable of efficiently infecting DHBV-infected PDHs. Exclusion was dependent on gene expression by the original infecting virus, since UV-irradiated DHBV was unable to mediate the exclusion of DHBV-ClaI. Using recombinant adenoviruses expressing DHBV proteins, we determined that the large surface antigen mediated exclusion. The large surface antigen is known to cause down-regulation of a DHBV receptor, carboxypeptidase D (CPD). Receptor down-regulation is a mechanism of superinfection exclusion seen in other viral infections, and so it was investigated as a possible mechanism of DHBV-mediated exclusion. However, a mutant large surface antigen which did not down-regulate CPD was still capable of inhibiting DHBV infection of PDHs. In addition, exclusion of DHBV-ClaI did not correlate with a decrease in CPD levels. Finally, virus binding assays and confocal microscopy analysis of infected PDHs indicated that the block in infection occurs after internalization of the second virus. We suggest that superinfection exclusion may result from the role of the L surface antigen as a regulator of intracellular trafficking.

Study of the early steps of the Hepatitis B Virus life cycle

Hepatitis B virus (HBV) is a human pathogen, causing the serious liver disease. Despite considerable advances in the understanding of the natural history of HBV disease, most of the early steps in the virus life cycle remain unclear. Virus attachment to permissive cells, fusion and penetration through cell membranes and subsequent genome release, are largely a mystery. Current knowledge on the early steps of HBV life cycle has mostly come from molecular cloning, expression of individual genes and studies of the infection of duck hepatitis B virus (DHBV) with duck primary duck hepatocytes. However, considering of the difference of the surface protein of HBV and DHBV both in the composition and sequence, the degree to which information from DHBV applies to human HBV attachment and entry may be limited. A major obstacle to the study HBV infection is the lack of a reliable and sensitive in vitro infection system. We have found that the digestion of HBV and woodchuck hepatitis virus (WHBV) by protease V8 led to the infection of HepG2 cell, a cell line generally is refractory for their infection [Lu et al. J Virol. 1996. 70. 2277-2285 . Lu et al. Virus Research. 2001. 73(1): 27-4].. Further studies showed that a serine protease inhibitor Kazal (SPIK) was over expressed in the HepG2 cells. Therefore, it is possible that to silence the over expressed SPIK and thus to reinstate the activity of indispensable cellular proteases can result in the restoration of the susceptibility of HepG2 cells for HBV infection. The establishing a stable cell line for study of the early steps of HBV life cycle by silencing of SPIK is discussed.

In vitro and in vivo infectivity and pathogenicity of the lymphoid cell-derived woodchuck hepatitis virus

Woodchuck hepatitis virus (WHV) and human hepatitis B virus are closely related, highly hepatotropic mammalian DNA viruses that also replicate in the lymphatic system. The infectivity and pathogenicity of hepadnaviruses propagating in lymphoid cells are under debate. In this study, hepato- and lymphotropism of WHV produced by naturally infected lymphoid cells was examined in specifically established woodchuck hepatocyte and lymphoid cell cultures and coculture systems, and virus pathogenicity was tested in susceptible animals. Applying PCR-based assays discriminating between the total pool of WHV genomes and covalently closed circular DNA (cccDNA), combined with enzymatic elimination of extracellular viral sequences potentially associated with the cell surface, our study documents that virus replicating in woodchuck lymphoid cells is infectious to homologous hepatocytes and lymphoid cells in vitro. The productive replication of WHV from lymphoid cells in cultured hepatocytes was evidenced by the appearance of virus-specific DNA, cccDNA, and antigens, transmissibility of the virus through multiple passages in hepatocyte cultures, and the ability of the passaged virus to infect virus-naive animals. The data also revealed that WHV from lymphoid cells can initiate classical acute viral hepatitis in susceptible animals, albeit small quantities (approximately 10(3) virions) caused immunovirologically undetectable (occult) WHV infection that engaged the lymphatic system but not the liver. Our results provide direct in vitro and in vivo evidence that lymphoid cells in the infected host support propagation of infectious hepadnavirus that has the potential to induce hepatitis. They also emphasize a principal role of the lymphatic system in the maintenance and dissemination of hepadnavirus infection, particularly when infection is induced by low virus doses.